1. Field of the Invention
This invention relates to an energy subtraction processing method, wherein energy subtraction processing is accurately carried out on all of image signal components of image signals representing images. This invention also relates to an apparatus for carrying out the method.
2. Description of the Prior Art
Techniques for photoelectrically reading out a radiation image, which has been recorded on a photographic film, in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, and the X-ray image is read out from the X-ray film and converted into an electric signal (i.e., an image signal). The image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Further, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In the radiation image recording and reproducing systems wherein recording media, such as X-ray film or stimulable phosphor sheets are used, subtraction processing techniques for radiation images are often carried out on image signals detected from a plurality of radiation images of an object, which have been recorded on the recording media.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals, which components represent the image information recorded at corresponding sampling points (i.e., picture elements) in the radiation images, are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order for the image of a specific structure (for example, a blood vessel) of an object to be extracted from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure (for example, a blood vessel) of the object is enhanced by the injection of contrast media. In the latter method, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy distributions. Specifically, an object is exposed to several kinds of radiation with different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object, is changed after it has been irradiated onto one of a plurality of radiation image recording media, after which the radiation impinges upon the second radiation image recording medium. In this manner, a plurality of radiation images are thereby obtained in which different images of a specific structure are embedded. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure. The subtraction process is carried out with Formula (3) shown below. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037. EQU Dproc=Ka.multidot.H-Kb.multidot.L+Kc (3)
wherein Dproc represents the subtraction image signal obtained from the subtraction process, Ka and Kb represents the weight factors, Kc represents the bias component, H represents the image signal representing the radiation image recorded with the radiation having a high energy level, and L represents the image signal representing the radiation image recorded with the radiation having a low energy level.
In the aforesaid energy subtraction processing, when an object is exposed to radiation having a predetermined energy distribution in the course of recording radiation images of the object, different parts of the object exhibit different levels of radiation transmittance. Also, the object exhibits a lower level of radiation transmittance with respect to the low energy components of the radiation than the high energy components thereof. Therefore, as the radiation passes through the object, the energy distribution of the radiation shifts to the high energy side as a whole. Such a phenomenon is referred to as the "beam hardening." The extent of the shift of the energy distribution varies for different parts of the object.
As described above, in the aforesaid energy subtraction processing, such characteristics are utilized that a tissue of an object exhibits different levels of radiation transmittance with respect to radiation with different energy distributions. From the energy subtraction processing, a subtraction image is obtained in which only the patterns of desired tissues of the object have been extracted or emphasized. Therefore, if the beam hardening phenomenon occurs to different extents for different parts of the object, the problems will occur in that patterns of unnecessary tissues are erased completely and only the patterns of desired tissues are embedded in a certain region of the subtraction image, whereas patterns of unnecessary tissues are not erased completely in a different region of the subtraction image. As a result, a subtraction image having good image quality cannot be obtained.
The difference in the extent of the beam hardening phenomenon at different parts of the object has a correlation with the image density in a radiation image (i.e. the value of the image signal representing the radiation image). Accordingly, the applicant proposed a novel method for forming an energy subtraction image in U.S. Pat. No. 5,210,415. With the proposed method, in the course of carrying out the subtraction process on the image signal components of a plurality of image signals, which image signal components represent corresponding picture elements in the plurality of the radiation images, the value of each of the parameters for the subtraction process in Formula (3) (i.e., the weight factor Ka for the image signal representing the radiation image recorded with the radiation having a high energy level, the weight factor Kb for the image signal representing the radiation image recorded with the radiation having a low energy level, and the bias component Kc) is changed for different parts of each radiation image in accordance with the value of the image signal representing the radiation image.
Specifically, with the proposed method for forming an energy subtraction image, the parameters Ka, Kb, and Kc in Formula (3) are respectively set to be functions Ka(Dorg), Kb(Dorg), and Kc(Dorg), which take values changing in accordance with the value of the image signal Dorg. In such cases, the subtraction image signal Dproc is calculated with Formula (4) shown below. EQU Dproc=Ka(Dorg).multidot.H-Kb(Dorg).multidot.L+Kc(Dorg) (4)
As the image signal Dorg, according to which the values of the parameters for the subtraction process are changed, it is possible to employ an image signal Dorg (i.e., the image signal representing the radiation image recorded with the radiation having a high energy level, or the image signal representing the radiation image recorded with the radiation having a low energy level), which is selected from the plurality of the image signals representing the plurality of the radiation images subjected to the subtraction process.
Alternatively, as the image signal Dorg, according to which the values of the parameters for the subtraction process are changed, an image signal Dmean may be employed, which is calculated from the plurality of the image signals (i.e., the image signal representing the radiation image recorded with the radiation having a high energy level, and the image signal representing the radiation image recorded with the radiation having a low energy level), and which represents the mean-level image of the plurality of the radiation images subjected to the subtraction process. The mean-level image of the plurality of the radiation images includes less noise components than the plurality of the radiation images. Therefore, in cases where the values of the parameters are changed in accordance with the values of the image signal representing the mean-level image of the plurality of the radiation images, adverse effects of the noise components upon the subtraction process can be minimized.
The functions of the parameters, which take values changing in accordance with the image signal Dorg, are determined experimentally in accordance with the portion of the object, the image of which is recorded, the region of interest in the radiation image, the conditions under which the image recording operation is carried out, or the like.
By way of example, in cases where each of the radiation images subjected to the subtraction process is a chest image and the lung field pattern in the image is taken as the region of interest, it is desired that subtraction process be carried out as uniformly as possible with respect to the region of interest. However, the lung field pattern contains rib patterns. If Formula (4) is applied to the image signal components representing the rib patterns or the image signal components affected by radiation noise superposed upon the rib patterns, it will often occur that the parameters do not take uniform value with respect to the region inside of the lung field pattern. In such cases, there is the risk that uniform subtraction image cannot be obtained with respect to the region inside of the lung field pattern.
Therefore, as the image signal Dorg, according to which the values of the parameters in Formula (4) are changed, an unsharp mask signal Lu should preferably be employed. The unsharp mask signal Lu is obtained by setting an unsharp mask constituted of a picture element matrix, which has a size of N columns.times.N rows and has its center at the picture element represented by the image signal Dorg, in a two-dimensional array of picture elements. The unsharp mask signal Lu represents the mean value of the image signal values corresponding to the picture elements located within the unsharp mask and is calculated with Formula (5) shown below. EQU Lu=(.SIGMA.Dorg)/N.sup.2 ( 5)
wherein .SIGMA.Dorg represents the sum of the image signal values representing the picture elements located within the unsharp mask. The subtraction process is then carried out with Formula (6) shown below. EQU Dproc=Ka(Lu).multidot.H-Kb(Lu).multidot.L+Kc(Lu) (6)
In such cases, the adverse effects of the rib patterns, noise, or the like, can be reduced.
However, in cases where the values of the parameters are changed in accordance with the unsharp mask signal Lu, which is calculated with Formula (5), the problems occur in that, with respect to a region, in which the image density changes sharply, an artifact due to density blurring occurs in the subtraction image.
FIG. 6 is an explanatory view showing an X-ray image of the chest. FIG. 7 is a graph showing signal value profiles taken along scanning line Y--Y' in the image of FIG. 6. Specifically, as for the X-ray image of the chest shown in FIG. 6, the original image signal Dorg corresponding to the region (scanning line Y--Y'), which extends across the edge of the lung field pattern, changes in the pattern indicated by the thick solid line in FIG. 7. Also, the unsharp mask signal Lu obtained with the unsharp mask constituted of a picture element matrix, which has a size of N columns.times.N rows, changes in the pattern indicated by the thin solid line in FIG. 7. In cases where the parameter Ka is a function of the input signal value as illustrated in, for example, FIG. 8, a difference of .vertline.Ka(a)-Ka(b).vertline. occurs in the value of the parameter Ka with respect to a certain picture element x between when the parameter value is set in accordance with the original image signal Dorg and when it is set in accordance with the unsharp mask signal Lu.
As illustrated in FIG. 9, if the difference occurs in the parameter for the subtraction process, density blurring will occur in the vicinity of the edge in the subtraction image, and an artifact will thereby be formed. Therefore, the image quality of the subtraction image cannot be kept high.